In many applications it is desired to monitor the rotation of a shaft or an axle around the longitudinal axis of the shaft or axle, by detecting the angle of rotation. It is known to detect the angle of rotation using a magnet assembly that is fixed to the shaft or axle, so as to co-rotate therewith. Such a magnet assembly produces a magnetic field that rotates along a circle that is concentric with the axis of rotation of the shaft or axle. To detect an angle of rotation, known angle sensors measure magnetic field components (e.g. x-y-component) and compare the measured magnetic field components to provide a value of the rotation angle, typically by using geometrical functions of the ratio of the two components. Thus, an angle sensor system is capable to uniquely determine the rotation angle (between 0 and 360°) at each rotation angle position of the axis from the at least two magnetic field components distinguished for example from speed sensors utilizing for example pole wheels who are capable of determining only increments of fractional angle amounts.
In an on-axis angle detection system, a non-rotationally symmetric button magnet may be placed at the end of the shaft. Magnetic sensors placed near the axis of rotation may be used to detect an angle of rotation.
For some applications, however, this known arrangement is not suitable, usually because of space limitations or other factors in the environment of the rotating shaft. Then, one or more field detectors may be situated around the rotating shaft or axle on or within a so-called reading radius (reading circle), in order to detect the magnetic field of the rotating magnet assembly, and thus to identify the magnitude of the rotating magnetic field at the detection locations. Such an angle sensor is known as an “off-axis” angle sensor, or an out of shaft angle sensor, because the sensor elements are not situated on the rotation axis of the shaft or axle.
In both types of angle sensing systems, Hall probes or magneto-resistive (MR) sensors may be used in order to detect the magnetic field and its orientation. Examples of such sensors are described in U.S. Pat. No. 8,779,760.
In general, known off-axis angle detection systems are not as precise as known on-axis systems. It is typical for an on-axis system to be able to detect the angle with an error of less than 1°, whereas known off-axis systems typically have a detection error of approximately 5°. A primary reason for the lower precision of off-axis systems is the sensitivity of such systems to manufacturing tolerances.
Such manufacturing tolerances include geometric tolerances, such as the positional error of the magnet on the shaft, as well as the respective positions of the sensor elements of the read circuit, both with respect to radial position and tilting of the shaft or axle. Errors that are inherent in the sensor elements themselves, such as zeroing (calibration) errors, gain and linearity errors may also contribute to the lower precision, as well as an inability to precisely match the sensitivities of individual sensors to each other, when multiple sensors are used in a sensing arrangement.